Fire Protection Glossary — 109 Terms

Short, verifiable definitions of 109 core fire protection terms. The definitions follow the terminology of NFPA, EN and FM Global documents; the binding text is always the standard itself.

Sprinklers and heads

Term	Definition
Sprinkler	An automatic water discharge device that opens with heat and distributes water in a defined pattern.
K-factor	The discharge coefficient of a sprinkler, relating flow to pressure in Q = K√P.
RTI	Response Time Index, the coefficient measuring how quickly a sprinkler responds to heat, in (m·s)^0.5.
Quick response (QR)	The fast-responding sprinkler class with an RTI of 50 or less.
Standard response	The class with an RTI above 80, preferred for storage ceiling applications.
Pendent sprinkler	A sprinkler hanging down from the pipe and discharging downwards.
Upright sprinkler	A sprinkler mounted above the pipe, facing upwards.
Sidewall sprinkler	A wall-mounted sprinkler discharging in a half-round pattern to one side.
Concealed sprinkler	A pendent sprinkler hidden behind a decorative cover plate that drops away with heat.
ESFR	Early Suppression Fast Response, a high K-factor quick-response sprinkler designed to suppress a storage fire from ceiling level.
CMSA	Control Mode Specific Application, a large-drop control mode sprinkler listed for defined storage arrangements.
CMDA	Control Mode Density Area, the classic control mode sprinkler designed by the density-area method.
In-rack sprinkler	An intermediate level sprinkler placed within racking, in the flue space.
Water shield	A metal disc of at least 0.075 m diameter protecting an in-rack sprinkler's thermal element from water above.
Baffle	A 200 mm by 150 mm shield placed between two sprinklers to prevent cold soldering.
Cold soldering	Where water from an operating sprinkler cools a neighbour and prevents it operating.
Glass bulb	The glass capsule that bursts as its liquid expands with heat, opening the sprinkler.
Fusible link	A mechanical sprinkler trigger held by solder that melts at a defined temperature.
Deflector	The plate that spreads water leaving a sprinkler into its designed pattern.
Spare sprinkler cabinet	A cabinet holding spare heads of every type and temperature used, plus the special wrench.

System types

Term	Definition
Wet pipe system	A sprinkler system whose pipework is permanently filled with pressurised water.
Dry pipe system	A system whose pipework is filled with compressed air and fed through a dry alarm valve.
Alternate system	A system operated wet in summer and dry in winter.
Pre-action system	A system where water enters the pipe only after independent detection operates.
Double interlock pre-action	A pre-action type requiring both detector operation and loss of pipe pressure.
Deluge system	A system with all heads open, where every head discharges when the valve opens.
Water spray system	A system distributing water through nozzles for equipment cooling and exposure protection.
Water mist system	A suppression system using very fine droplets for cooling and vapour dilution.
Wet alarm valve	The control valve in a wet system that senses flow and triggers the alarm.
Water motor gong	A mechanical alarm driven by water passing the valve, requiring no electricity.
Test and drain valve	The valve assembly at the end of a system for flow testing and draining.
Flow switch	A device that senses water flow in the pipe and generates an electrical alarm signal.
Tamper switch	A device monitoring that a valve stays open, signalling a fault when it closes.
Backflow preventer	A device preventing contamination of the potable supply by system water.

Design and hydraulics

Term	Definition
Design density	The water flow per unit of protected area, expressed in mm/min or gpm/ft².
Area of operation	The hydraulic calculation area covered by the sprinklers assumed to operate together.
Most remote area	The hydraulically most demanding sprinkler group, where the calculation starts.
Hazen-Williams equation	The relation calculating pipe friction loss using a C coefficient.
C factor	The Hazen-Williams constant representing internal pipe roughness.
Equivalent length	Expressing valve and fitting losses as an equivalent straight pipe length.
Velocity pressure	The dynamic component of total pressure arising from flow velocity.
Hydraulic balancing	Equalising the pressures of branches meeting at a node.
Hazard class	The LH, OH, HHP and HHS classification by fire load and rate of spread.
LH	Light Hazard, areas of low fire load such as offices, schools and hospital rooms.
OH1–OH4	Ordinary Hazard, the four sub-classes of medium-risk production and commercial areas.
HHP	High Hazard Process, the class for high-risk production activities.
HHS	High Hazard Storage, the high-risk storage class, expressed as categories I to IV.
ST1–ST6	The storage configuration types in EN 12845 (bulk, palletised, racked and so on).
Commodity class	In NFPA 13, Class I to IV and plastics grouping by the fire behaviour of material and packaging.
Flue space	The vertical clearance left between pallets in racking, through which heat and water pass.
Longitudinal flue	The gap between two back-to-back rack rows, required to be at least 0.15 m.
Transverse flue	The gap between side-by-side pallets, required to be at least 0.10 m.
Pallet stop	A mechanical stop preventing a pallet being pushed in and closing the flue.
Clearance	The distance between the top of the stored goods and the sprinkler deflector.

Pumps and water supply

Term	Definition
Fire pump	A listed centrifugal pump providing the system's required pressure and flow.
Jockey pump	An auxiliary pump maintaining system pressure by making up small leaks.
NPSHa	The net positive suction head the installation provides at the pump suction.
NPSHr	The net positive suction head the pump requires to avoid cavitation.
Cavitation	Bubble formation and collapse when suction pressure falls to vapour pressure, eroding the impeller.
Shut-off head	The pressure the pump produces at zero flow.
Overload point	Operation at 150% of rated flow, the NFPA 20 performance point.
Positive suction	Where the water source sits above the pump axis; the preferred arrangement.
Suction lift	Where the water source sits below the pump axis; restricted by NFPA 20.
Pressure maintenance	The control logic watching system pressure and starting the pump when it falls.
No automatic shutdown	The requirement that a fire pump be stopped manually rather than automatically.
Diesel fire pump	A pump with its own fuel tank and batteries, independent of mains electricity.
Water storage tank	A dedicated tank providing the volume required for the system's design duration.
Reduced capacity tank	A tank arrangement kept smaller by a controlled infill supply.
Suction pit	A recess at the tank floor allowing the pump to draw the last water.
Anti-vortex plate	A plate arrangement preventing air being drawn into the suction.

Detection and alarm

Term	Definition
Point smoke detector	A detector measuring smoke density at a single point.
Beam detector	A linear detector measuring attenuation of a beam between transmitter and receiver.
Aspirating detector	A highly sensitive system drawing air through pipes and analysing it centrally.
Heat detector	A detector operating on fixed temperature or rate of temperature rise.
Linear heat cable	A cable detector sensing heat at any point along its length.
Flame detector	A detector sensing UV or IR radiation to identify flame very quickly.
Addressable system	A fire detection architecture where each device is identified by its own address.
Conventional system	An architecture grouping devices by zone, without individual addresses.
Fire alarm panel	The central control unit monitoring detection devices and managing the alarm.
Sounder	A siren or loudspeaker giving audible warning in a fire.
Visual alarm device	A flashing warning for noisy areas or where users may be hearing impaired.
Alarm receiving centre	A remote centre where alarm signals are continuously monitored.
Cross-zoning	Logic requiring confirmation from two separate detectors to reduce false alarms.

Other suppression systems

Term	Definition
Clean agent	A non-conductive gaseous agent leaving no residue after discharge.
Inert gas	Argon, nitrogen or a mixture that suppresses by diluting oxygen.
Hold time	The period the agent concentration is retained in the enclosure after discharge.
Door fan test	A test measuring enclosure tightness to verify the hold time.
Pressure relief damper	A damper venting the overpressure produced during agent discharge.
NOAEL	The highest agent concentration at which no adverse physiological effect is observed.
AFFF	Aqueous film-forming foam concentrate used on hydrocarbon fires.
AR-AFFF	An alcohol-resistant AFFF usable on polar solvents as well.
F3 foam	Fluorine-free foam concentrate, the PFAS-free generation of product.
Foam proportioner	A device blending foam concentrate into water at the correct ratio.
Kitchen hood system	A wet chemical suppression system for fryer and range cooking oil fires.
Hose reel cabinet	First-response equipment housing a hose reel or lay-flat hose.
Fire hydrant	An external water take-off point fed from the main, for fire brigade connection.
Fire brigade connection	An external inlet allowing a fire appliance to pump into the system.

Inspection, testing and maintenance

Term	Definition
Main drain test	A flow test through the main drain checking the condition of the water supply.
Alarm test	A weekly test verifying operation of the alarm valve and water motor gong.
Inspector's test valve	A test valve at the most remote point simulating the flow of one sprinkler.
Annual flow test	An annual test verifying the pump's flow-pressure curve on site.
Dry system trip test	A test measuring the opening time of the dry alarm valve.
Obstruction investigation	An examination for sediment, corrosion and blockage inside the pipework.
Microbiologically influenced corrosion	Pitting corrosion of the pipe wall caused by bacterial activity (MIC).
Acceptance test	The set of tests verifying the system matches the design before handover.
Hydrostatic test	Holding the pipework under water pressure above its design pressure.
Flushing	Clearing foreign material from the pipework by flow before commissioning.
Impairment	Temporarily taking the system out of service for maintenance, with compensating measures.
Hot work permit	A controlled permit system for spark-producing work such as welding and cutting.

How to use this page

This page is a quick reference list. For a detailed reading of what a term means in design, see the glossary articles; to see the difference between two standards, see the comparison articles.

Frequently Asked Questions

Are the definitions on this page binding?

No. They are summaries following NFPA, EN and FM Global terminology; the binding text is always the relevant standard itself.

What is the difference between K-factor and design density?

The K-factor is the discharge coefficient of a single sprinkler. Design density is the flow per unit of protected area; together they determine the sprinkler selection.

What is the difference between control mode and suppression mode?

Control mode limits the fire and cools its surroundings, leaving extinguishment to the fire brigade. Suppression mode (ESFR) aims to suppress the fire directly from ceiling level.

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